Update tox to 3.7.0

[pyup-bot]

This PR updates tox from 3.6.1 to 3.7.0.

The bot wasn't able to find a changelog for this release. Got an idea?

Links

• PyPI: https://pypi.org/project/tox
• Docs: https://tox.readthedocs.org/

[codecov]

Codecov Report

Merging #19 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #19   +/-   ##
======================================
  Coverage    9.76%   9.76%           
======================================
  Files           5       5           
  Lines         389     389           
======================================
  Hits           38      38           
  Misses        351     351

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